Sustainable Hydrogen Production

C H A P T E R

1 Sustainable Hydrogen Production: An Introduction S. Venkata Mohan1, 3, Ashok Pandey2 1

Bioengineering and Environmental Sciences Lab, CEEFF, CSIR-Indian Institute of Chemical Technology (CSIR-IICT), Hyderabad, India; 2Centre for Innovation and Translational Research, CSIR-Indian Institute of Toxicology Research, Lucknow, India; 3Academy for Scientific and Industrial Research (AcSIR), Hyderabad, India

1. ESSENTIALS OF ENERGY Increasing gap between the energy requirement and inability to replenish needs from the limited energy sources has resulted in a steep increase in fossil fuel utilization. This has put a severe strain on the depleting fossil fuels and resulted in an increase in pollution levels across the globe. Increasing levels of greenhouse gases (GHGs) from the fossil fuels combustion, in turn, aggravated the perils of global warming. Combustion of fossil fuels adds about 6 Gigatons (Gton ¼ 109 tons) of carbon per year in the form of CO2 to the atmosphere [1]. In the past few decades, human activities have released organic carbon that is equivalent to that accumulated over millions of years. The bulk emissions mainly come from motor vehicles, which alone accounts for more than 70% of the global carbon monoxide (CO) and 19% of CO2 emissions [2]. Petroleum products were used to power when kerosene began to replace the whole oil near about 150 years ago [3]. According to the current consumption trends, oil, natural gas, and coal reserves may get depleted in another 125 years [4]. Global primary energy consumption increased by 1% in 2016, following a growth of 0.9% in 2015 and 1% in 2014 with a concomitant CO2 emission increase by 0.1% in 2016 [5]. Concerns about climate change from GHG emissions and limited availability of global oil reserves instigated interest in the development of clean and renewable energy alternatives to satisfy the growing energy demands. Diversification of energy sources is an essential requirement in the present day energy scenario [6]. Development of renewable, carbon-neutral, alternative, and eco-friendly fuels is of paramount importance to fulfill the burgeoning energy demands. Global energy markets are in transition and the energy mix is shifting towards cleaner, lower carbon fuels, driven by environmental needs and technological advances [5].

Biohydrogen, Second Edition https://doi.org/10.1016/B978-0-444-64203-5.00001-0

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Copyright © 2019 Elsevier B.V. All rights reserved.

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1. SUSTAINABLE HYDROGEN PRODUCTION: AN INTRODUCTION

Although the share of renewable energy within total energy remains small, at around 4%, it accounted for almost a third of the increase in primary energy last year. The coming decades will see a profound energy transformation and the quest for tapping renewable energy from sun, wind, biomass, biowastes, and geothermal power.

2. HYDROGEN 2.1 Energy Carrier Hydrogen gas (H2) is a promising and important energy carrier that could play a significant role in the reduction of GHG emissions [7,8]. H2 is a carbon-neutral energy carrier and on combustion produce water. High energy yield (122 kJ/g), which is 2.75 folds greater than that of hydrocarbon fuels, makes H2 as an ideal energy. High energy density, low to nonexistent generation of pollutants, and higher efficiency of conversion to usable power makes H2 a carbon-neutral energy carrier in the realm of fossil fuel depletion and environmental pollution. In 1970, John Bockris proposed “Hydrogen Economy,” which represents a system that promotes H2 as a potential fuel to solve some of the negative effects of using hydrocarbons. It is assured that H2-based economy would be less polluting than a fossil fuel-based economy. High electrochemical reactivity of H2 makes it ideal for fuel cells application in the presence of suitable catalysts. In fact, the sun’s energy also results from the nuclear fusion of H2. Liquid H2 was used to propel space shuttles and rockets. Fuel cells with H2-powered shuttle’s electrical systems producing pure water, was used by the crew as drinking water. Fuel cells basically reverse the electrolysis process, where hydrogen and oxygen are combined to produce electricity. H2-powered vehicles are now on roads and hydrogen is believed to be highly promising fuel for both stationary and transportation uses which will have huge potential in the future.

2.2 Other Applications The current merchant (purchased from H2 producers) and captive (consumed by H2 producer) market of H2 is mostly used in food production, oil refining, fertilizer manufacture, and metals treatment [8]. Its applications in petroleum recovery, chemical and pharmaceutical industry, refining, aerospace and fuel cells, metal production and fabrication, fertilizer production (fixation of nitrogen from the air in the Haber ammonia process), hydrogenation of fats and oils, glass purification, semiconductor manufacturing, methanol production, power plants (as coolant), cryogenics, etc., is well-established. Hydrogen is also used as hydrogenating and reducing agent, food additive, shielding gas, rotor coolant, etc. High demand for H2 is now with petroleum refinery and ammonia production [9].

2.3 Production Routes When traced by time, H2 was first produced (artificially) in the early 16th century by Robert Boyle through a reaction between iron and acid. In the 17th century, Henry Cavendish recognized H2 gas as a discrete substance and named it as “flammable air” or “water-

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former.” Antoine Lavoisier in 1783 gave the element the name “Hydrogen” (in Greek hydro means water and genes meaning creator) [10]. Hans Gaffron, in 1939, noticed that typical green algae named Chlamydomonas reinhardtii generally grown in pond scum would occasionally change from producing oxygen to hydrogen, what we now called as “Biohydrogen.” H2 does not occur in elemental form even though it is the most common element on the Earth. Industrial production of H2 is majorly from fossil sources through steam reforming of natural gas or methane supplementing marginally from the energyintensive water-splitting electrolysis process and as by-product from some industrial processes. Global H2 production currently exceeds one billion m3/day, of which 48% is produced from natural gas, 18% from coal, 30% from oil (often on-site in refineries), and the remaining 4% by water electrolysis [11]. In conjunction with steam reforming, high-purity H2 can also be produced by wateregas shift (WGS) reaction, which is one of the important industrial reactions especially used for ammonia synthesis. Partial oxidation of coal, heavy residual oils, and other low-value refinery products is second in production capacity after steam reforming of natural gas [8,9]. Catalytic partial oxidation, autothermal reforming, gasification thermal decomposition, and pyrolysis were other thermochemical routes used for H2 production [12]. Steam gasification of coal is the third largest production technology in terms of production capacity [9]. H2 production from fossil fuels is accompanied with the production of GHGs, viz., CO2, CH4, etc. [11]. At present, H2 synthesis through biological routes from waste/biomass is an emerging interest globally due to its sustainable nature. Global H2 production market is valued at USD 115 billion in 2017 and is projected to be worth USD 154 billion by 2022 (Markets and Markets, 2017). Global H2 production volume is forecasted to grow by the compound annual growth rate of 6.07%. Asia and Oceania region is the largest market with 39% of global production share in 2010 and accounts for a production of 21 million metric tons of hydrogen [9]. The global H2 generation market is driven by factors, such as government regulations for the desulfurization of petroleum products and rising demand for hydrogen as a transportation fuel [13].

3. TRANSITION TOWARDS BIOENERGY Looming energy crisis, decreasing fossil fuel resources, and climate change concerns coupled with high oil prices have garnered global attention towards the development of renewable, alternative, eco-friendly, and carbon-neutral fuels to fulfill the burgeoning energy demands. An exciting and sustainable alternative for the fossil fuels is bioenergy, which can save the planet from the brink of environmental catastrophe and defend the energy crisis. Bioenergy is deemed to have the potential to provide renewable and carbon-neutral energy through sustainable routes. They offer a strategy to diversify energy sources to reduce supply risks and also help to promote domestic rural economies [14]. Bioenergy derived from microorganisms is of great interest in the present world’s energy scenario due to its renewability. Microorganisms has flexible and diverse metabolic machinery to synthesize various forms of biobased products including bioenergy/fuels.

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1. SUSTAINABLE HYDROGEN PRODUCTION: AN INTRODUCTION

4. GENESIS OF BIOLOGICAL H2 PRODUCTION Biohydrogen is a transitory and natural by-product of various biochemical reactions driven by microbes. Generation of H2 gas either by the biological machinery or through thermochemical treatment of biomass can be defined as “biohydrogen” (Fig 1.1). Incidentally, thermochemically produced H2 is also being termed as biohydrogen due to the usage of biomass as substrate/feedstock. On the contrary, biological routes for biohydrogen production are available pertaining to acidogenesis/anaerobic/fermentation (dark), photobiological, enzymatic and electrogenic mechanisms. In the past two decades, research fraternity showed significant interest on biological routes of H2 production. Remarkable research on biohydrogen in both basic and applied fields has been illustrated in the passing decade. Based on the light dependency, biological H2 production processes can be further classified into light-dependent photosynthetic processes and light-independent (dark) fermentation. In another way, photobiological process can again be classified either into fermentation or photosynthetic process depending on the carbon source and the biocatalyst used. Lightdependent processes can occur through biophotolysis of water using green algae and cyanobacteria via direct and indirect biophotolysis or via photofermentation mediated by photosynthetic bacteria (PSB). Microalgae and cyanobacteria undergo indirect and direct biophotolysis by utilizing inorganic CO2 to produce H2 in the presence of sunlight and water, while PSB manifests H2 production through photofermentation by consuming a wide variety of substrates ranging from inorganic to organic acids in the presence of light. On the other hand, dark fermentation process confines to anaerobic metabolism where anaerobic microorganism (mostly acidogenic bacteria [AB]) metabolically generate H2 through acetogenic process along with the generation of volatile fatty acids (VFAs) and CO2. H2 production occurs in the absence of O2 during fermentation. In the electrogenic approach, microbial electrolysis is a strategy where external potential will be applied to the microbial cells for increasing H2

FIGURE 1.1 Schematic illustration representing various routes of biohydrogen production.

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production. Synthetic enzymes mediate in vitro H2 production, which is one of the fascinating routes envisaged by the scientists. The metabolism and biochemistry involved vary significantly with the biological routes based on the function of biocatalyst used, operating conditions adapted, microenvironment employed and substrate/feedstock used.

5. BIOHYDROGENESIS 5.1 Photosynthetic Machinery Solar energy can be converted into biochemical energy with the help of photosynthetic apparatus to molecular H2 [15]. This is one of the promising approaches helpful for sustainable energy generation. Photosynthetic mechanism of H2 production was known for over 70 years [16]. It is imperative to note that the photosynthetic microbial community has contributed to the chemical evolution of the earth and could potentially be a source of renewable H2 in the future too [17]. However, the taxonomy of H2-producing microorganisms, i.e., hydrogenogens have been classified mainly into three physiologically distinct groups of microbes, which includes prokaryotes and eukaryotes varying from unicellular green algae [16], cyanobacteria to PSB (anoxygenic) [18,19]. Based on the photosynthetic organism, any of the two diverse photosynthetic machineries (anoxygenic or oxygenic) functions for H2 production (Figs. 1.2e1.4). The mechanism of H2 production is marginally different between these two processes [19e21]. The light energy supply electrons (e) generates proton gradient either from a parallel Photosystem II (PSII)-independent process originating from the breakdown of the starch molecule (indirect photolysis; Fig 1.3) or water splitting reaction (direct photolysis; Fig 1.2). In green algae and cyanobacteria, light functions as a driving force for PSII resulting in the production of reducing powers , which are used for the splitting of water into e, protons (Hþ), and O2 through direct photolysis via oxygenic photosynthesis [22e24]. In both the cases, reduced ferredoxin (Fd) serves as an electron donor for [FeeFe]hydrogenases (HydAs). The reducing powers are transferred to the chlorophyll a dimer (P700) residing in Photosystem I (PSI). P700 gets excited by light absorption and yields e

FIGURE 1.2 Mechanism of direct photolysis involved in oxygenic photosynthesis during H2 production.

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FIGURE 1.3 Mechanism of indirect photolysis involved in the oxygenic photosynthesis during H2 production.

FIGURE 1.4

Mechanism of cyanobacterial H2 production mediated through heterocyst (Nitrogen fixation

reaction).

at a potential of 1.32 V (vs NHE, Normal Hydrogen Electrode) [25]. Finally, e are conducted through the internal electron transport chain to the ironesulfur clusters located at the acceptor site of PSI at a potential of 0.45 V (Jordan et al., 1998). In both the photolysis processes, reducing equivalents gets reduced to H2 by the [FeeFe]-HydA. Hydrogenase acts as

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Hþ/e release valve by recombining Hþ (from the medium) and e (from reduced Fd) to produce H2. The supply of reducing equivalents derived either directly by photosynthetic water splitting (driven by PSII) or indirectly from the degradation of organic molecules decides the hydrogenase activity. Microalgae are prominent for their extremely active [FeeFe]-HydA enzyme with the ability of high conversion (12%e14%) of solar energy to molecular H2 [26] from the oxidation of water molecules to generate H2 in their chloroplast [27]. However, during the direct biophotolysis, oxygen (generated as a by-product of the function of PSII) is a dominant suppressor of HydA enzyme. Therefore direct biophotolysis for H2 production can be operated for short periods of time upon the start of illumination, before accumulating O2 and inactivates the H2 production process. Hþ and e generated from water in the indirect process are stored in the form of starch during photosynthesis, which will be fed into the plastoquinone pool and onto HydA via PSI under certain stress conditions [28]. Through the Calvin cycle, cyanobacteria and green algae accumulate reserve compounds. These reserve compounds provide energy to carry out the cellular metabolism when photosynthesis is shut off at night time [22]. The transition from aerobic to anaerobic conditions is often accompanied by the cessation of photosynthetic light reaction and generation of excess reductant, which is finally turned into H2 by hydrogenase. Two O2-sensitive [FeeFe]-HydAs (HydA1 and HydA2) are induced to catalyze the reduction of Hþ to molecular H2 under anaerobic conditions [24]. The processes of photosynthesis and H2 evolution are coupled only by indirect process and are time delayed [22]. Cyanobacteria has one additional mechanism for H2 production, i.e., via heterocyst during N2 fixation. H2 production can also be catalyzed by nitrogenase during nitrogen fixation and the process is extremely O2 sensitive. A specific mechanism for protecting nitrogenase from O2 through localization of nitrogenase in the heterocyst of filamentous cyanobacteria [29] is observed, which is responsible for NH3 and H2 production (Fig 1.4). PSB uses sunlight as a source of energy and produces H2 and CO2 by degrading the organic molecules under anaerobic conditions (Fig 1.5). PSB does not require water as a

FIGURE 1.5

Anoxygenic H2 production mechanism in PSB.

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source of e so it can easily circumvent the oxygen-sensitivity issue that adversely affects the [FeeFe]-HydA. PSB can utilize both the visible (400e700 nm) and near-infrared (700e950 nm) regions of the solar spectrum. Anoxygenic photosynthesis by PSB is advantageous for H2 production due to the absence of oxygen, which is a scavenging molecule of reducing equivalents. PSB neither uses water as an e source nor produces O2 photosynthetically [30]. Light absorption by a dimer of bacteriochlorophyll molecules instigates the reaction forming bacteriopheophytin (BPh) [31] (Fig. 1.4). The e transfer proceeds from BPh to quinine pool (QA) and then to the cytochrome subunit of the reaction center generating a Hþ gradient, which drives the ATP (Adenosine Triphospate) generation and finally gets reduced to H2 [30,32]. The efficiency of light energy conversion to H2 by PSB is much higher than that by cyanobacteria because of less quantum of light energy requirement than the water photolysis [29,33,34]. The ability of PSB to trap energy over a wide range of the light spectrum and its versatility in utilizing various substrates makes photofermentation process more feasible and viable. In both the oxygenic and anoxygenic photosynthesis, the reduced Fd serves as an e donor for the [FeeFe]-HydAs. Along with the hydrogenases, nitrogenase also plays a major role for the H2 production in all the photosynthetic systems. The main drawback of these two enzymes is that they get inhibited by oxygen liberated from the photolysis of water. Holding back oxygen is essentially required to enhance H2 production. Reduction in sulfate concentration during algal growth showed decrement in photosynthesis (90% oxygen production reduction), sufficient to allow the HydA enzyme to continue diverting e towards Hþ to yield H2 for a longer period [35]. Molecular structure of photosynthetic membrane redirects photosynthetically generated reducing equivalents from PSI to H2 production [36]. PSI is a robust nanometer-scale molecular photovoltaic device [37] located on a nonappraised region of thylakoid membrane. Platinum (Pt) can be precipitated on the stromal side of the photosynthetic thylakoid membrane at the site of e emergence from the PSI reaction center [38]. Chloroplast thylakoids are capable of trapping this e facilitating simultaneous photoevolution of H2 and oxygen [39]. Photosynthetic membranes have the capability to transform with metals other than Pt, such as osmium and ruthenium. Chemical platinization of PSI has no inhibitory effect on excitation-transfer dynamics and/or reaction-center-pigment [40] (Lee et al., 1995). Surface metallization at functional nanoscale towards the reducing ends of isolated PSI was reported by substituting negatively charged hexachloroplatinate ([PtCl6]2) for negatively charged Fd, the naturally occurring water-soluble electron carrier in photosynthesis [36]. Enzymatic H2 production via visible light-induction with Pt colloid using the photosensitization of Mg chlorophyll a (Mg Chl-a) was also reported [41]. Mg Chl-a acts as the effective photosensitizer with absorption maximum at 670 nm. Most of the photobiological process for H2 production has major fundamental limitations and practical engineering issues that need to be resolved prior to practical development [42,43].

5.2 Acidogenesis/Dark Fermentation Microbial fermentation generates reducing powers (FADH, NADH, etc.) from metabolism that gets reoxidized with simultaneous generation of biological energy molecules (ATP) in the presence of a terminal electron acceptor (TEA) during respiration. Oxygen is a strong

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5. BIOHYDROGENESIS

TEA existing in biological system that helps in ATP generation during aerobic respiration by simultaneous regeneration of reducing powers. On the contrary, anaerobic respiration has the ability to utilize a wide range of compounds, viz., NO3, SO4 2 , organic and inorganic compounds, etc., as TEA with their simultaneous reduction and regeneration of reducing powers. ATP generation is not assured during these processes as the energy from the reducing equivalents (Hþ and e) will be utilized towards reduction reaction with electron acceptor but not transferred to the bonding between ADP (Adenosine Diphosphate) and inorganic phosphate to generate ATP. Glycolysis is the primary metabolic pathway where substrate gets converted to pyruvate, a central molecule of the microbial fermentation. During aerobic fermentation, pyruvate transforms to CO2 and H2O with simultaneous generation of reducing powers, which will further help in energy generation. On the contrary, pyruvate under anaerobic fermentation has a diverse fate based on operating conditions. Pyruvate enters the acidogenic pathway and generates VFAs, viz., acetic acid, propionic acid, butyric acid, malic acid, etc., in association with the generation and re-utilisation of H2 (Eq.1.1-1.5). C6 H12 O6 þ 2H2 O/2CH3 $COOH þ 2CO2 þ 4H2

ðAcetic acidÞ

(1.1)

C6 H12 O6 /CH3 $CH2 $CH2 $COOH þ 2CO2 þ 2H2

ðButyric acidÞ

(1.2)

C6 H12 O6 þ 2H2 /2CH3 $CH2 $COOH þ 2H2 O

ðPropoinic acidÞ

C6 H12 O6 þ 2H2 /COOH$CH2 $CH2 O$COOH þ CO2 C6 H12 O6 /CH3 $CH2 OH þ CO2

ðMalic acidÞ

ðEthanolÞ

(1.3) (1.4) (1.5)

Both facultative and obligate AB can catalyze H2 production from organic substrates [44e55]. Facultative anaerobes convert pyruvate to acetyl-CoA and formate by the action of pyruvate formate lyase and further H2 is produced by formate hydrogen lyase (VardarSchara et al., 2008). Fig. 1.6 illustrates the biochemical route of dark fermentative pathway for H2 production. While obligate anaerobes convert pyruvate to acetyl-CoA and CO2 through pyruvate Fd oxidoreductase and this oxidation process requires the reduction of Fd [45,56]. The proton reducing reactions facilitates the generation of H2, a common fermentation by-product during electron-acceptor limited microbial processes [57]. Interconversion of metabolites takes place during substrate degradation in anaerobic fermentation, which increases the availability of reducing equivalents in the cell. The Hþ from redox mediators (NADH:Nicotinamide Adenine Dinucleotide/FADH:Flavin Adenine Dinucleotide Hydrogen) gets detached in the presence of NADH-dehydrogenase enzyme and gets reduced to H2 in the presence of the HydA enzyme with the help of e donated by the oxidized Fd (cofactor). While mobile carrier proteins (quinine and cytochrome C) and membranebound protein complexes (NADH dehydrogenase and cytochrome b-c1) facilitate the e transport through quinone (Q) pool (Fig 1.6). The continuous interconversions of Q and Hþ (from the cytosol) to QH2, and QH2 to Q and Hþ facilitates the e transfer to the cytochrome bc1 complex and further to the cytochrome aa3. Finally, the e gets transferred to

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FIGURE 1.6

1. SUSTAINABLE HYDROGEN PRODUCTION: AN INTRODUCTION

Schematic illustration of substrate conversion and H2 production mechanism during dark fermen-

tation process.

the iron-containing protein Fd from the cytochrome aa3. This reduced Fd donates e to the active site component of HydA enzyme, which reduces the Hþ with this e producing H2 [46]. Nitrogenase and HydA are the two vital enzymes involved in fermentative H2 production by catalyzing the reversible reduction of Hþ to H2 [42]. Both enzymes contain the complex metal clusters at their active site with diverse subunits. [FeeFe]-HydA and [Ni-Fe]-HydA are the two important enzymes involved in microbial H2 production. [FeeFe]-HydA removes the excess reducing equivalents. [NieFe]-HydA also called as uptake hydrogenase involves in quinine pool, where e are used directly or indirectly for NAD (Nicotinamide-Adenine Dinucleotide)/NADP(Nicotinamide-Adenine Dinucleotide Phosphate) reduction. Hydrogenases catalyze the reduction of Hþ to H2 by oxidizing a suitably strong reductant including the natural electron carrier proteins Fd and/or flavodoxin, which have redox potentials near that of the H2 electrode (420 mV). NADPH (Nicotinamide Adenine Dinucleotide Phosphate Hydrogen) is too positive (320 mV) to serve as a direct reductant of hydrogenase, except in hyperthermophiles where the H2 redox potential is near this potential. Nitrogenase enzyme contains two-component protein systems, MoeFe protein and Fe protein, which involves in H2 production process [58]. Nitrogenase uses Mg-ATP and e to reduce a variety of substrates during H2 production. Dehydrogenase is another important enzyme involved in the interconversion of metabolites and the transfer of Hþ between metabolic intermediates

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through redox reactions using several mediators (NADþ, FADþ, etc.) [47,59]. Both hydrogenase and dehydrogenase functions are important to maintain Hþ equilibrium in the cell and to reduce them to H2. Physiologically distinct groups of microorganisms responsible for a series of interrelated biochemical reactions, viz., hydrolysis, acidogenesis, acetogenesis, and methanogenesis, manifest the conversion of organic substrate to its end products [60]. The complex organic compounds get degraded to monomers during hydrolysis by hydrolytic microorganisms. Further, these monomers will be fermented by AB to generate a mixture of low molecular weight organic acids associated with H2 during acidogenesis (Eq. 1.5). The reversible interconversion of acetate production from H2 and CO2 by acetogens and homoacetogens can also be considered for H2 production. Finally, the acetoclastic methanogens convert these organic acids to CH4 and CO2 through methanogenesis [60e63]. Acetogens grows in synergistic association with the hydrogenotrophic methanogens (H2 consuming) and maintain H2 partial pressure low enough to allow acidogenesis to become thermodynamically favorable by interspecies H2 transfer. Henceforth, the methanogenic activity needs to be suppressed to make H2 as a sole metabolic by-product.

5.3 Electrically Driven Microbial electrolysis cell (MEC) represents an alternative electrically driven H2 production process that contains combined metabolism of microorganisms interlinked with electrochemistry [64,65]. MEC more or less resembles a microbial fuel cell where the basic difference exists with the requirement of small input of external potential to facilitate the conversion of biodegradable material into H2. The Hþ transferred to cathode were reduced to form H2 in presence of e transferred from the anode (as an electron sink) under the applied voltage, which is essentially required to cross the endothermic barrier to form H2 gas. The standard redox potential for the reduction of Hþ to H2 is 0.414 V. The potential greater than 0.11 V in addition to that generated by bacteria (0.3 V) facilitates good H2 production at the cathode [66]. This approach provides a route for extending H2 production to surpass the endothermic barrier imposed by the end products from microbial fermentation and the potential required is relatively low compared to theoretically applied voltage of 1.23 V for water electrolysis [67]. Acetate (0.279 V) can be converted to H2 (0.414 V) in a cathodic reaction against the thermodynamic gradient, with the application of a relatively small voltage (0.135 V). In practice, relatively high voltage than this is required due to over potentials created by physicochemical and microbial factors [68]. Application of external voltage also results in the selective growth of electrochemically active microbes on anode which can effectively sink e [69]. MEC documented more than 90% of H2 recovery as against 33% with dark fermentation process [66]. MEC is also termed with different nomenclatures as bioelectrochemically assisted microbial reactor or electrohydrogenesis or biocatalyzed electrolysis cell. MEC showed the capability to convert a wide variety of soluble organic matter to H2 or methane with simultaneous waste treatment [70,71]. The application of MEC can also be seen with the usage of acidogenic effluents rich in fatty acids as primary substrate for additional H2 production associated with simultaneous treatment of wastewater [72]. H2 production from waste/wastewater is renewable with low energy consumption when

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compared to the conventional water electrolysis where H2 production can be achieved by high energy consumption. High product (H2) recovery and substrate degradation than the dark fermentation process are some of the potential benefits that make MEC as an alternate process. Nature of electrode materials, biocatalyst, membrane, nature of substrate and its loading rate, applied potential, and configuration play a vital role in the performance of MEC. MEC was initially operated in dual chamber and later shifted to single chamber. Dualchamber system allowed separate capture of H2 (cathode) and CO2 (anode) and prevents fouling of the cathode by anodic bacteria. On the contrary, separation led to inhibitory pH changes, since there is potential acidification of the anode chamber due to the production of Hþ and basification of the cathode chamber from proton consumption. Lower the applied potential eliminates the membrane to create a single-chamber MEC [73]. Eliminating the membrane, attenuated pH energy loss and ohmic energy loss [74,75], which were significant for a dual-chamber MEC [67]. More recently, single-chamber MECs were operated to eliminate some of the inherent disadvantages [76], which significantly reduces the internal resistance. Extensive research is carried out on MEC using waste/wastewater as substrate [77,78]. Integrating MEC with other processes is also gaining much attention [79]. There are a number of challenges to be addressed before MEC can be applied on a practical level. Little applied voltage with an elevated current density will be an essential challenge for moving bench scale MEC to commercial application [80e83]. Biohydrogen production by integrating the wastewater treatment with MEC is economically feasible and recent interest among the research fraternity.

5.4 In Vitro Hydrogenesis In vitro synthetic enzymatic pathway for directing e to the hydrogenase is one of the interesting and alternative routes for the synthesis of H2. This approach facilitates the improvement of typically low yields encountered with in vivo process where the microorganism has its own metabolic requirements that need to be satisfied along with the accumulation of inhibitory fermentative by-products. In vitro H2 production helps to achieve higher yields as the cellular metabolic needs are eliminated [84]. Synthetic biology infers the engineering and biological entities by assembling the interchangeable parts of the natural biology into the systems that function unnaturally [85,86]. About 11.6 mol H2 per mol glucose-6phosphate (G6P) is produced by the enzymes coupled dehydrogenase (purified from the bacterium Pyrococcus furiosus) and enzymes of oxidative pentose phosphate pathway (PPP), which uses NADP þ as the e acceptor. Biohydrogen is the main product of this pathway unlike that produced by intermediate metabolic pathways of bacterial fermentation [87]. In extension to this, Zhang et al. (2007) used a synthetic pathway consisting of 13 purified enzymes from the PPP coupled with the [Ni-Fe]-HydA (P. furiosus) and achieved 8.35 mol H2 per mole of G6P. When all the enzymes of the oxidative branch of the PPP cycle apart from 6-phosphogluconolactonase were used with the hydrogenase, NADPþ and G6P about 97% of the maximum stoichiometric yield of H2 from G6P was attained [88]. The yield of biohydrogen from the oxidative PPP represents the reaction almost to completion as a result of the hydrogenase oxidizing NADPH as soon as it is formed and sweep out. Type of synthetic

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enzymes may vary with the type of substrate used for the H2 production. H2 production from starch and water was also reported, employing 13 different synthetic enzymes (Zhang et al., 2007). A new enzymatic approach for the production of H2 using Fd-NADPH-reductase to transfer e from NADPH to Fd, which after oxidation delivers the e to a [FeeFe]-HydA resulting in H2 production was demonstrated by Smith et al. (2012). This alternative in vitro pathway enables the utilization of the fastest known [FeeFe]-HydAs and activates electron delivery by the native electron donor. Even though, higher conversion yields are achievable with the existing in vitro methodologies compared to fermentative processes, using multiple purified enzymes for a full-scale industrial process would be prohibitively expensive [84].

5.5 Thermochemical Process Thermochemical treatment of biomass is only the nonbiological process pertaining to H2 production. Thermochemical processes involve either gasification or pyrolysis (heating biomass in the absence of oxygen) to produce a H2-rich stream of gas known as “syngas” (a blend of hydrogen and CO). Thermally assisted chemical reactions that release the hydrogen from hydrocarbons or water involved in thermochemical processes for H2 production [89e91]. Thermochemical process can utilize a broad range of feedstock. Gasification of biomass in the presence of oxygen and/or steam at temperatures above 1000 K undergoes partial oxidation and/or steam reforming reactions yielding gas and char product [92]. H2, CO, CO2, and CH4 were the subsequent reduced products of the char. This process is more favorable for H2 production than pyrolysis. Gasification by partial oxidation exists over 150 years. Low temperature (<1000 C) gasification yields sufficiently great amount of hydrocarbon while at higher temperature the yield of syngas contains almost no hydrocarbons. Pyrolysis makes thermal decomposition at a temperature of 650e800 K (1e5 bar) of biomass in absence of air to yield liquid oils, solid charcoal, and gaseous compounds [92]. Hydrogen can be produced directly through fast or flash pyrolysis if both high temperature and sufficient volatile phase residence time are provided. Pyrolysis followed by reforming of biooil and gasification has received a significant amount of interest as these provide improved quality fuel product [93]. Gasification followed by reforming of the syngas and pyrolysis (fast) followed by reforming of the carbohydrate fraction of the biooil are the two major thermochemical routes used for H2 production [94]. WGS is used along with pyrolysis and gasification processes to convert the reformed gas into H2 and pressure swing adsorption is used to purify the product. Supercritical water condition (pressures > 221 bar; temperatures > 647 K) in the absence of oxygen can convert biomass into fuel gases, which can be easily separated from the water phase by cooling to ambient temperature (Navarro et al., 2009). The cost of H2 from steam methane reforming is cheaper than the H2 production from supercritical water gasification of wet biomass [94]. Today, the most widely used and least expensive technology to produce H2 is catalytic steam reforming of natural gas [95]. Steam reforming converts hydrocarbons to CO and H2 driven by steam addition either through thermal reforming at high temperatures (>1100 C) or catalytic reforming (>650 C) over nickel-based catalysts [96]. The large-scale production of H2 from natural gas and other available hydrocarbons through catalytic reforming

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processes remains the cheapest source of H2. Still gasification is the most mature technology, which results in a high production of H2, but it needs integrated demonstration plants at sufficiently large scale including catalytic gas upgrading. Pyrolysis processes for H2 production are at present in the smaller scale demonstration phase while supercritical water gasification is still in early stage of development and research. Hydrogen production involving reforming technologies produces large amounts of CO2, which has an impact on global warming [92]. Applying reforming methods to alternative renewable precursors is one way to reduce CO2 emissions.

6. WASTE AS RENEWABLE FEEDSTOCK FOR BIOHYDROGEN PRODUCTION Rejected materials from anthropogenic activities and natural sources are recently being considered as potential feedstock/substrate for harnessing renewable bioenergy. Finding new ways to produce useful/value-added products from treatment reducing the treatment cost of waste/wastewater and has been gaining importance due to its sustainable nature. In the contemporary energy scenario, environmental scientists are gradually shifting their focus from “pollution control” to “resource exploitation from waste.” Biological processes are generally preferred to treat wastewater as they are technically feasible, simple, economical, and eco-friendly. Biological approaches also facilitate the conversion of negative valued organic waste to useful forms of energy while simultaneously achieving the objective of the pollution control. Enormous quantity of waste/wastewater is available, which is composed of reasonably good biodegradable carbon fraction associated with inherent net positive energy. The regulatory need for their treatment prior to disposal makes them as an ideal commodity to produce H2 from the anaerobic treatment. H2 generation through biological routes has instigated considerable interest by utilizing wastes as a potential source due to its sustainable nature and further opening up a new avenue for the utilization of renewable and inexhaustible energy sources [97e99]. Biohydrogen generation from renewable wastewater associated through treatment simultaneously reduces the overall effluent treatment cost by creating additional revenue and makes the whole process environmentally sustainable [100]. Municipal and industrial wastewater along with the waste generated from agriculture and food-processing industries contain enough amount of organic load, which can be beneficially tapped if appropriately utilized. Combination of the biohydrogen production with the existing Effluent Treatment Plant is the futuristic goal envisaged with the usage of wastewater as primary feedstock. Biologically derived organic material and their residue, such as wood and wood waste, food processing waste, aquatic plants, algae, agricultural crops, and their waste byproducts, constitute a large source of biomass, which can also be used as fermentable substrate [101]. Biomass precursors derived from plant crops, agricultural residues, woody biomass, etc., are being used as feedstock for generating H2 by both thermochemical and biological routes [102]. Different types of lignocellulosic/agricultural residues, such as sugar cane and sweet sorghum bagasse, corn stalks and stover, fodder maize, wheat straw, etc., and forestry residues, such as wood trimmings, have been studied as potential renewable feedstocks for dark fermentative biohydrogen production [103]. Unlike wastewater, cellulosic

7. BIOCATALYST FOR BIOHYDROGEN PRODUCTION

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materials or solid wastes require initial pretreatment to make organic fractions soluble and bioavailable to the microorganism for its metabolism. Highly crystalline and waterinsoluble nature of cellulose makes recalcitrant to the hydrolysis. Direct microbial assimilation of cellulosic materials facilitates low H2 yields. Development of novel and effective cellulase enzymes, optimization and improvement of cellulase system, as well as engineering approaches on cellulose pretreatment, and saccharification are gaining increasing interest to overcome this limitation. Combinations of chemical, mechanical, and enzymatic pretreatment methods were used to pretreat cellulose-based feedstock to usable carbohydrates. Techniques, viz., high temperature, high or low pH, hydrolytic enzymes, microwaves, ultrasound, radiation, and pulsed electric fields, were applied for this purpose [104]. On the contrary, wastewater represents a readily available carbon source. Dark fermentation is one of the most common processes for biohydrogen production since waste utility was specifically found with dark fermentation process. Fermentative H2 production is relatively less energy intensive and more environmentally sustainable due to the utilization of waste and wastewater as substrate and due to operational feasibility at ambient temperature and pressure [99]. Exploitation of wastewater as substrate for H2 production with simultaneous wastewater treatment will lead to open a new avenue for the utilization of renewable and inexhaustible energy sources. In conjunction with the wastewater treatment, this process is capable of solving two issues, viz., reduction of pollutants in waste and the generation of a clean alternative fuel [105e107]. Certain inherent limitations, viz., low substrate conversion efficiency, accumulation of carbon-rich acid intermediates, drop in system pH, etc., still exists with the process, which needs considerable attention prior to process upscaling [55,108]. At present, basic and applied research mainly focusing on the way to gain more insight into the process understanding towards establishing optimized conditions. Especially after starting the usage of wastewater as substrate, a great deal of attention was noticed on the application of mixed consortia as biocatalyst for the acidogenic fermentation process for H2 production.

7. BIOCATALYST FOR BIOHYDROGEN PRODUCTION Selection of inoculum or appropriate biocatalyst significantly influences the metabolic endproduct formation, which is also true with the H2 evolution. Diverse groups of microorganisms, viz., photosynthetic (heterotrophic and autotrophic), anaerobic, and microalgae, are efficient in producing H2 by taking advantage of their specific metabolic route under defined conditions. Bacteria capable of producing H2 are reported to widely exist in natural environments. Thermophiles, methanogens, obligate anaerobes, and few facultative anaerobes involve in metabolic H2 production. Initial research on biohydrogen was mostly confined towards the usage of pure cultures as biocatalyst with the defined substrate. Especially after starting the usage of wastewater as substrate, a great deal of attention was noticed in the acidogenic fermentation process by using mixed consortia as biocatalyst taking advantage of its diverse biochemical functions. It is believed that the application of mixed consortia as biocatalyst is one of the promising and practical options for the scaling up of the biohydrogen technology especially when wastewater is used as substrate. Mixed cultures are usually preferred because of diverse biochemical functions, stability, operational flexibility,

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possibility of using a broad range of substrates, and moreover restrict the requirement of sterile conditions [47,60,109]. From an engineering point of view, producing H2 by mixed culture offers lower operational cost and ease of control in concurrence to the possibility of using waste as feedstock [47]. The feasibility of H2 production with typical anaerobic mixed consortia is limited as it gets rapidly consumed by methanogens [110,111]. Shifting or regulating the metabolic pathway towards acidogenesis and inhibiting methanogenesis facilitate higher H2 yields [111]. Pretreatment of biocatalyst prior to fermentation plays an important role in the selective enrichment of mixed consortia towards shifting the metabolic function towards acidogenesis [110,112]. Application of pretreatment to parent inoculum facilitates the selective enrichment of AB capable of producing H2 as the end-product with the simultaneous prevention of hydrogenotrophic methanogens [58,108,110,111,113]. Physiological differences between H2 producing AB and H2 uptake bacteria (methanogens; MB) forms fundamental basis for the methods used for the preparation of H2 producing inoculum [61,112]. Pretreatment also blocks competitive growth and coexistence of other H2 consuming bacteria.

8. CONCLUSIONS AND PERSPECTIVES Use of bioenergy reduces GHG emissions [114], though the extent of reduction depends on the production technology [115]. At present, bioenergy production is relatively expensive compared to the fossil fuels [114] and the introduction of more efficient techniques for their production as well as life cycle assessment of the process may help in the overall cost reduction. Federal governments around the globe are encouraging bioenergy sector in the form of subsidies to commercialize bioenergy production. Research on biohydrogen production at both basic and applied areas is currently in the developing phase. Initial interest in biohydrogen research was much visible with the photobiological route using specific strains and defined medium. Inhibitory effect of O2 on the HydA and nitrogenase enzymes [116] and low rates of H2 production are few inherent demerits linked with the photobiological process. The O2 sensitivity of H2 production, combined with the competition between hydrogenases and NADPH-dependent CO2 fixation are the main limitations for the commercialization of photosynthetic water splitting coupled to hydrogenase-catalyzed H2 production [117]. Requirement of high activation energy to drive hydrogenase and low solar conversion efficiencies are also considered as major limitations. Overcoming oxygen sensitivity of HydA enzymes, ensuring adequate efficiency when capturing and converting solar energy, and outcompeting other metabolic pathways for photosynthetic reductants are few of the key technical challenges [27]. A key issue regarding the functional advantage of the photosynthetic process especially with the evolution of H2 is being focused to eliminate the O2 sensitivity. Operating at ambient temperatures and pressures is gaining importance as a practically viable method among the biological routes for light-independent dark fermentation. Due to the feasibility of utilizing a broad range of substrates including waste/wastewater with mixed cultures as biocatalysts, significant progress was witnessed in dark fermentation process in the last decade. Dark fermentation process is relatively less energy intensive, technically much simpler, requires low operating costs, and is more stable and robust [44,56,118e121]. The process simplicity, efficiency, and lesser footprints are some of the

8. CONCLUSIONS AND PERSPECTIVES

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striking features of the dark fermentation process, which makes it practically more feasible for the mass production of H2 [52,121]. Inspite of the advantages, low substrate conversion efficiency, low yields and production rates, and fatty acid-rich wastewater generated from the acidogenic process are major barriers to the practical implementation of this technology [53,61]. The accumulation of acidogenic by-products causes a sharp drop in the pH resulting in the inhibition of the fermentation process. The undissociated soluble metabolites can permeate through the cell membrane of H2-producing bacteria and then dissociate in the cell leading to physiological imbalance resulting in the cell lysis especially at higher concentrations [109]. When more reduced organic compounds, such as lactic acid, propionic acid, and ethanol, are produced as fermentation products, H2 yield is lower because these represent the end products of metabolic pathways that bypass the major H2-producing reaction [62,63]. As the NADH oxidation by NADH-Fd oxidoreductase is inhibited under standard conditions and proceeds only at very low partial pressures of H2, in practice, yields are lower [60]. However, biological limitations, such as H2-end-product inhibition and acid or solvent accumulation, limit the molar yield. Fundamental understanding of the factors to overcome imitations is imperative and important. Process parameters optimization is essential for upscaling of the technology. Optimization of operational factors and process engineering govern the performance of any biological system and have considerable influence on the acidogenic H2 production. Even under optimum conditions about, 50% of the organic fraction remains in the wastewater. Apart from lower conversion efficiency, the unutilized organic fraction usually remaining as a soluble fermentation product (short-chain fatty acids), is a major concern to be resolved. Cost of H2 production is also one of the issues to be resolved prior to scaling up the technology. Hydrogen can also be used as additive along with methane as biohythane [61,122] or with compressed natural gas (CNG). This enables to use the existing infrastructure of CNG and also reduce the emissions significantly. Environmental and economic concerns suggest that it is advisable to use the residual carbon fraction of acidogenic outlet for additional resource recovery [99,123]. Integrated approaches were extensively studied to overcome some of the persistent limitations by utilizing fatty acid-rich wastewater as the primary substrate integrating in biorefinery format with the secondary unit operation, viz., methanogenesis, acidogenic fermentation [124], photobiological process [32,43,125], microbial electrolysis [78,82,83,126], bioplastics production [127,128] and lipid synthesis using microalgae [129,130], etc., were evaluated with diverse degree of success. Acidogenic effluent rich in organic acids (acetic, propionic and butyric acid) also acts as a good chelating agent which can potentially solubilize the soil phosphate and helps in plant growth which significantly contributes towards organic farming [131]. These integration approaches facilitate reduction in waste load with the advantage of value addition in the form of product recovery. Short-chain fatty acids (as VFA) are one of the major metabolites produced along with H2 and CO2 during the acidogenic fermentation. Due to its significant productivity concomitant to the H2 evolution, extraction of fatty acids as biobased products in biorefinery approach will significantly reduce the overall cost of H2 production [53,54]. Metabolic engineering approaches, viz., expression of heterologous proteins including hydrogenases, to overcome thermodynamic barriers, rerouting metabolism to achieve more complete substrate degradation, and increased electron flux for proton reduction, are being developed for higher H2 yields [68]. To establish an environmentally sustainable

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biohydrogen technology, multidisciplinary research approach is vital. With the documented advances in the performance over the past 20 years, it can be presumed that a level for practical application can be achieved in coming days with sustainability at the forefront.

Acknowledgments SVM gratefully acknowledge the Ministry of New and Renewable Energy (MNRE), Government of India, for financial support in the form of National Mission Mode Project on Hydrogen Production through Biological Routes (No. 103/131/2008-NT).

References [1] IPCC, IPCC guidelines for national greenhouse gas inventories, in: H.S. Eggleston, L. Buendia, K. Miwa, T. Ngara, K. Tanabe (Eds.), Prepared by the National Greenhouse Gas Inventories Programme, IGES, Japan, 2006. [2] J. Goldemberg, Environmental and ecological dimensions of biofuels, in: Proceedings of the Conference on the Ecological Dimensions of Biofuels, Washington, DC, March 10, 2008. [3] M. Grayson, Nature outlook-Biofuels, Nature 474 (2011) S1. [4] BPSR, (2008). http://www.ief.org/whatsnew/Pages/BPStatisticalReview.aspx. [5] BPSR, (2017) (https://www.bp.com/content/dam/bp/en/corporate/pdf/energy-economics/statisticalreview-2017/bp-statistical-review-of-world-energy-2017-full-report.pdf. [6] M.R. Nouni, Hydrogen energy and fuel cell technology: recent developments and future prospects in India, Renew. Energy-Akshay Urja 5 (5) (2012) 10e14. [7] K. Christopher, R. Dimitrios, A review on exergy comparison of hydrogen production methods from renewable energy sources, Energy Environ. Sci. 5 (2012) 6640e6651. [8] T. Lipman, An Overview of Hydrogen Production and Storage Systems with Renewable Hydrogen Case Studies, Energy Efficiency and Renewable Energy Fuel Cell Technologies Program, 2011 (US DOE Grant DE-FC3608GO18111 A000). [9] Markets and Markets, Hydrogen Generation Market-by Merchant & Captive Type, Distributed & Centralized Generation, Application & Technology-Trends & Global Forecasts (2011e2016), 2011. Report code: EP1708, http://www.marketsandmarkets.com/Market-Reports/hydrogen-generation-market-494.html. [10] A. Stwertka, A Guide to the Elements, Oxford University Press, 1996, pp. 16e21. [11] W. Jong, Sustainable hydrogen production by thermochemical biomass processing, in: R.B. Gupta (Ed.), Hydrogen Fuel: Production, Transport, and Storage, Taylor & Francis Group, LLC, 2009. [12] W. Chen, Y. Syu, Hydrogen production from water gas shift reaction in a high gravity (Higee) environment using a rotating packed bed, Int. J. Hydrogen Energy 35 (19) (2010) 10179e10189. [13] Markets and Markets, Hydrogen Generation Market by Generation and Delivery Mode (Captive and Merchant, Application, Technology, and Region - Global Forecasts to 2022, 2017. https://www. marketsandmarkets.com/Market-Reports/hydrogen-generation-market-494.html. [14] International Energy Agency (IEA), Key World Energy Statistics 2008, OECD/IEA, Paris, 2008. [15] P. Jordan, P. Fromme, H.T. Witt, O. Klukas, W. Saenger, N. Krauss, Three dimensional structure of cyanobacterial photosytstem I at 2.5 Å resolution, Nature 411 (2001) 909e917. [16] H. Gaffron, J. Rubin, Fermentative and photochemical production of hydrogen in algae, J. Gen. Physiol. 26 (1942) 219e240. [17] L.C. Burow, D. Woebken, B.M. Bebout, P.J. McMurdie, S.W. Singer, J. Pett-Ridge, L. Prufert-Bebout, A.M. Spormann, P.K. Weber, T.M. Hoehler, Hydrogen production in photosynthetic microbial mats in the Elkhorn Slough estuary, Monterey Bay, ISME J. 6 (4) (2012) 863e874. [18] H. Gest, M.D. Kamen, Studies on the metabolism of photosynthetic bacteria IV. Photochemical production of molecular hydrogen by growing cultures of photosynthetic bacteria, J. Bacteriol. 58 (1949) 239e245. [19] O. Kruse, J. Rupprecht, K.P. Bader, T.S. Hall, P.M. Schenk, G. Finazzi, B. Hankamer, Improved photobiological H2 production in engineered green algal cells, J. Biol. Chem. 280 (2005) 34170e34177.

REFERENCES

19

[20] O. Kruse, B. Hankamer, Microalgal hydrogen production, Curr. Opin. Biotechnol. 21 (3) (2010) 238e243. [21] L.L. Beer, E.S. Boyd, J.W. Peters, M.C. Posewitz, Engineering algae for biohydrogen and biofuel production, Curr. Opin. Biotechnol. 20 (2009) 264e271. [22] H. Krassen, A.A. Schwarze, B. Friedrich, B. Ataka, O. Lenz, J. Heberle, Photosynthetic hydrogen production by a hybrid complex of photosystem I and [NiFe]-Hydrogenase, ACS Nano 3 (12) (2009) 4055e4061. [23] S.I. Allakhverdiev, V. Thavasi, V.D. Kreslavski, S.K. Zharmukhamedov, V.V. Klimov, S. Ramakrishna, D.A. Los, M. Mimuro, H. Nishihara, R. Carpentier, Photosynthetic hydrogen production, J. Photochem. Photobiol. 11 (2010) 87e99. [24] A. Doebbe, The interplay of proton, electron, and metabolite supply for photosynthetic H2 production in Chlamydomonas reinhardtii, J. Biol. Chem. 285 (2010) 30247e30260. [25] L. Krabben, E. Schlodder, R. Jordan, D. Carbonera, G. Giacometti, H. Lee, A.N. Webber, W. Lubitz, Influence of the axial ligands on the spectral properties of P700 of photosystem I: a study of site-directed mutants, Biochemistry 39 (2000) 13012e13025. [26] A. Melis, Solar energy conversion efficiencies in photosynthesis: minimizing the chlorophyll antennae to maximize efficiency, Plant Sci. 177 (2009) 272e280. [27] A. Melis, T. Happe, Hydrogen production. Green algae as a source of energy, Plant Physiol. 127 (3) (2001) 740e748. [28] A. Melis, Photosynthetic H2 metabolism in Chlamydomonas reinhardtii (unicellular green algae), Planta 226 (2007) 1075e1086. [29] D. Vyas, H.D. Kumar, Nitrogen fixation and hydrogen uptake in four cyanobacteria, Int. J. Hydrogen Energy 20 (2) (1995) 163e168. [30] R.E. Blankenship, M.T. Medigan, C.E. Bauer (Eds.), Anoxygenic Photosynthetic Bacteria, Kluwer Academic Publishers, Dordresht. The Netherlands, 1995, pp. 1005e1028. [31] J.M. Berg, J.L. Tymoczko, L. Stryer, Biochemistry, fifth ed., W. H. Freeman & Co, New York, 2002, p. 380. [32] R. Chandra, S. Venkata Mohan, Microalgal community and their growth conditions influence biohydrogen production during integration of dark-fermentation and photo-fermentation processes, Int. J. Hydrogen Energy 36 (2011) 12211e12219. [33] K.A. Batyrova, A.A. Tsygankov, S.N. Kosourov, Sustained hydrogen photoproduction by phosphorusdeprived Chlamydomonas reinhardtii cultures, Int. J. Hydrogen Energy 37 (2012) 8834e8839. [34] A. Melis, Photosynthesis-to-fuels: from sunlight to hydrogen, isoprene, and botryococcene production, Energy Environ. Sci. 5 (2012) 5531e5539. [35] A. Melis, L. Zhang, M. Forestier, M.L. Ghirardi, M. Seibert, Sustained photobiological hydrogen gas production upon reversible inactivation of oxygen evolution in the green alga Chlamydomonas reinhardtii, Plant Physiol. 122 (2000) 127e133. [36] J.F. Millsaps, B.D. Bruce, J.W. Lee, E. Greenbaum, Nanoscale photosynthesis: photocatalytic production of hydrogen by platinized photosystem I reaction centers, Photochem. Photobiol. 73 (2001) 630e635. [37] I. Lee, J.W. Lee, A. Stubna, E. Greenbaum, Measurement of electrostatic potentials above oriented photosystem I reaction centers, J. Phys. Chem. B 104 (2000) 2439e2443. [38] E. Greenbaum, Platinized chloroplasts: a novel photocatalytic material, Science 230 (1985) 1373e1375. [39] E. Greenbaum, Interfacial photoreactions at the photosynthetic membrane interface: an upper limit for the number of platinum atoms required to form a hydrogen-evolving platinum metal catalyst, J. Phys. Chem. 92 (1988) 4571e4574. [40] J.W. Lee, I. Lee, P.D. Laible, T.G. Owens, E. Greenbaum, Chemical platinization and its effect on excitation transfer dynamics and P700 photooxidation kinetics in isolated photosystem I, Biophys. J. 69 (1995) 652e659. [41] Y. Saiki, Y. Amao, Visible light-induced enzymatic hydrogen production from oligosaccharides using Mg chlorophyll-a and platinumcolloid conjugate system, Int. J. Hydrogen Energy 29 (2004) 695e699. [42] P.C. Hallenbeck, J.R. Benemann, Biohydrogen- the microbiological production of hydrogen fuel, in Biotechnology, in: H.W. Doelle, Edgar J. Da Silva (Eds.), Encyclopedia of Life Support Systems(EOLSS), Developed under the Auspices of the UNESCO, Eolss Publishers, Oxford ,UK, 2009. [43] R. Chandra, G.N. Nikhil, S. Venkata Mohan, Single-stage operation of hybrid dark-photo fermentation to enhance biohydrogen production through regulation of system redox condition: evaluation with real-field wastewater, Int. J. Mol. Sci. 16 (2015) 9540e9556.

20

1. SUSTAINABLE HYDROGEN PRODUCTION: AN INTRODUCTION

[44] P.C. Hallenbeck, J.R. Benemann, Biological hydrogen production: fundamentals and limiting processes, Int. J. Hydrogen Energy 27 (2002) 1185e1193. [45] G. Vardar-Schara, T. Maeda, T.K. Wood, Metabolically engineered bacteria for producing hydrogen via fermentation, Microb. Biotechnol. 1 (2008) 107e125. [46] S. Venkata Mohan, Harnessing of biohydrogen from wastewater treatment using mixed fermentative consortia: process evaluation towards optimization, Int. J. Hydrogen Energy 34 (2009) 7460e7474. [47] S. Venkata Mohan, S. Srikanth, M. Lenin Babu, P.N. Sarma, Insight into the dehydrogenase catalyzed redox reactions and electron discharge pattern during fermentative hydrogen production, Bioresour. Technol. 101 (2010) 1826e1833. [48] G. Dinopoulou, R.M. Sterritt, J.N. Lester, Anaerobic acidogenesis of a complex wastewater kinetics of growth, inhibition, and product formation, Biotechnol. Bioeng. 31 (1988) 969e978. [49] D.W. Klein, L.M. Prescott, J. Harley, Microbiology, McGraw-Hill., New York, 2005. [50] S. Venkata Mohan, L. Agarwal, G. Mohanakrishna, S. Srikanth, A. Kapley, H.J. Purohit, P.N. Sarma, Firmicutes with iron dependent hydrogenase drive hydrogen production in anaerobic bioreactor using distillery wastewater, Int. J. Hydrogen Energy 36 (2011a) 8234e8242. [51] S. Venkata Mohan, G. Mohanakrishna, S. Srikanth, Biohydrogen production from industrial effluents (Chap 22), in: A. Pandey, C. Larroche, S.C. Ricke, C.-G. Dussap, E. Gnansounou (Eds.), Biofuels: Alternative Feedstocks and Conversion Processes, Academic Press, Imprint of Elsevier, Burlington, 2011b, ISBN 978-012-385099-7, pp. 499e524. [52] S. Venkata Mohan, P. Chiranjeevi, G. Mohanakrishna, A rapid and simple protocol for evaluating biohydrogen production potential (BHP) of wastewater with simultaneous process optimization, Int. J. Hydrogen Energy 37 (2012a) 3130e3141. [53] S. Dahiya, O. Sarkar, Y.V. Swamy, S. Venkata Mohan, Acidogenic fermentation of food waste for volatile fatty acid production along with co-generation of biohydrogen, Bioresour. Technol. 182 (2015) 103e113. [54] O. Sarkar, A.N. Kumar, S. Dahiya, K.V. Krishna, Y.D. Kumar, S. Venkata Mohan, Regulation of acidogenic metabolism towards enhanced short chain fatty acid biosynthesis from waste: metagenomic profiling, RSC Adv. 6 (2016a) 18641e18653. [55] O. Sarkar, B. Sai Kishore, S. Venkata Mohan, Acidogenesis driven by hydrogen partial pressure towards bioethanol production through fatty acids reduction, Energy 118 (2017) 425e434. [56] J.T. Kraemer, D.M. Bagley, Improving the yield from fermentative hydrogen production, Biotechnol. Lett. 29 (2007) 685e695. [57] E.L. Madsen, Environmental Microbiology: From Genomes to Biogeochemistry, Blackwell Publishing, Malden, MA, USA, 2008. [58] R.K. Goud, S. Venkata Mohan, Acidic and alkaline shock pretreatment to enrich acidogenic biohydrogen producing mixed culture: long term synergetic evaluation of microbial inventory, dehydrogenase activity and bioelectro kinetics, RSC Adv. 2 (2012a) 6336e6353. [59] S. Srikanth, S. Venkata Mohan, V. Lalit Babu, P.N. Sarma, Metabolic shift and electron discharge pattern of anaerobic consortia as a function of pretreatment method applied during fermentative hydrogen production, Int. J. Hydrogen Energy 35 (2010a) 10693e10700. [60] L.T. Angenent, K. Karim, M.H. Al-Dahhan, B.A. Wrenn, R. Domiguez-Espinosa, Production of bioenergy and biochemicals from industrial and agricultural wastewater, Trends Biotechnol. 22 (2004) 477e485. [61] O. Sarkar, S. Venkata Mohan, Pre-aeration of food waste to augment acidogenic process at higher organic load: valorizing biohydrogen, volatile fatty acids and biohythane, Bioresour. Technol. 242 (2017) 68e76. [62] S. Venkata Mohan, S. Srikanth, G.N. Nikhil, Augmentation of bacterial homeostasis by regulating in situ buffer capacity: significance of total dissolved salts over acidogenic metabolism, Bioresour. Technol. 225 (2017) 34e39. [63] S. Srikanth, S. Venkata Mohan, Regulating feedback inhibition caused by the accumulated acid intermediates during acidogenic hydrogen production through feed replacement, Int. J. Hydrogen Energ 39 (2014) 10028e10040. [64] H. Liu, S. Grot, B.E. Logan, Electrically assisted microbial production of hydrogen from acetate, Environ. Sci. Technol. 39 (2005) 4317e4320. [65] R. Rozendal, H.V.M. Hamelers, G.J.W. Euverink, S.J. Metz, C.J.N. Buisman, Principle and perspectives of hydrogen production through biocatalyzed electrolysis, Int. J. Hydrogen Energy 31 (2006) 1632e1640.

REFERENCES

21

[66] S. Cheng, B.E. Logan, Sustainable and efficient biohydrogen production via electrohydrogenesis, Proc. Natl. Acad. Sci. USA 104 (2007) 18871e18873. [67] R.A. Rozendal, H.V.M. Hamelers, R.J. Molenkmp, J.N. Buisman, Performance of single chamber biocatalyzed electrolysis with different types of ion exchange membranes, Water Res. 41 (9) (2007) 1984e1994. [68] P.C. Hallenbeck, M. Abo-Hashesh, D. Ghosh, Strategies for improving biological hydrogen production, Bioresour. Technol. 110 (2012) 1e9. [69] B. Wang, W. Wan, J. Wang, Effect of ammonia concentration on fermentative hydrogen production by mixed cultures, Biores. Technol. 100 (2009) 1211e1213. [70] D. Call, B.E. Logan, Hydrogen production in a single chamber microbial electrolysis cell lacking a membrane, Environ. Sci. Technol. 42 (2008) 3401e3406. [71] P. Clauwaert, W. Verstraete, Methanogenesis in membraneless microbial electrolysis cell, Appl. Microbiol. Biotechnol. 82 (2009) 829e836. [72] E. Lalaurette, S. Thammannagowda, A. Mohagheghi, et al., Hydrogen production from cellulose in a two-stage process combining fermentation and electrohydrogenesis, Int. J. Hydrogen Energy 34 (2009) 6201e6210. [73] H. Hu, Y. Fan, H. Liu, Hydrogen production using single chamber membrane-free microbial electrolysis cells, Water Res. 42 (2008) 4172e4178. [74] H.S. Lee, C.I. Torres, P. Parameswaran, B.E. Rittmann, Fate of H2 in an upflow single-chamber microbial electrolysis cell using a metal-catalyst-free cathode, Environ. Sci. Technol. 43 (2009) 7971e7976. [75] P. Clauwaert, R. Toledo, D.V. d. Ha, R. Crab, W. Verstraete, H. Hu, K.M. Udert, K. Rabaey, Combining biocatalyzed electrolysis with anaerobic digestion, Water Sci. Technol. 57 (2008) 575e579. [76] D.-W. Liang, S.-K. Peng, S.-F. Lu, et al., Enhancement of hydrogen production in a single chamber microbial electrolysis cell through anode arrangement optimization, Bioresour. Technol. 102 (2011) 10881e10885. [77] A. Escapa, M.I. San-Martin, A. Moran, Potential use of microbial electrolysis cells in domestic wastewater treatment plants for energy recovery, Front. Energy Res. 2 (2014) 19. [78] R.C. Wagner, J.M. Regan, S.E. Oh, Y. Zuo, B.E. Logan, Hydrogen and methane production from swine wastewater using microbial electrolysis cells, Wat. Res. 43 (2009) 1480e1488. [79] R.D. Cusick, P.D. Kiely, B.E. Logan, A monetary comparison of energy recovered from microbial fuel cells and microbial electrolysis cells fed winery or domestic wastewaters, Int. J. Hydrogen Energy 35 (2010) 8855e8861. [80] H.S. Lee, B.E. Rittmann, Significance of biological hydrogen oxidation in a continuous single-chamber microbial electrolysis cell, Environ. Sci. Technol. 44 (2010) 948e954. [81] G.N. Nikhil, G.V. Subhash, Y. Dileep, S. Venkata Mohan, Synergistic yield of dual energy forms through biocatalyzed electrofermentation of waste: stoichiometric analysis of electron and carbon distribution, Energy 88 (2015) 281e291. [82] J.S. Sravan, B. Sai Kishore, S. Omprakash, K.V. Krishna, S. Venkata Mohan, Electrofermentation of food wasteregulating acidogenesis towards enhanced fatty acids production, Chem. Eng. J. l (334C) (2018) 1709e1718. [83] K. Arunasri, J.A. Modestra, Y.D. Kumar, K.V. Krishna, S. Venkata Mohan, Polarized potential and electrode materials implication on electro-fermentative di-hydrogen production: microbial assemblages and hydrogenase gene copy variation, Bioresour. Technol. 200 (2016) 691e698. [84] P.R. Smith, A.S. Bingham, J.R. Swartz, Generation of hydrogen from NADPH using an [FeFe] hydrogenase, Int. J. Hydrogen Energy 37 (2012) 2977e2983. [85] D. Endy, Foundations for engineering biology, Nature 438 (2005) 449e453. [86] S.A. Benner, A.M. Sismour, Synthetic biology, Nat. Rev. Genet. 6 (2005) 533e543. [87] J. Woodward, M. Orr, K. Cordray, E. Greenbaum, Enzymatic production of biohydrogen, Nature 405 (2000) 1014e1015. [88] Y.H.P. Zhang, B.R. Evans, J.R. Mielenz, R.C. Hopkins, M.W.W. Adams, High-yield hydrogen production from starch and water by a synthetic enzymatic pathway, PLoS ONE 2 (2007) p456. [89] B. Yildiz, M.S. Kazimi, Efficiency of hydrogen production systems using alternative nuclear energy technologies, Int. J. Hydrogen Energy 31 (2006) 77e92. [90] M. Agarwal, J. Tardio, S. Venkata Mohan, Pyrolysis of activated sludge: energy analysis and its technical feasibility, Bioresour. Technol. 178 (2015a) 70e75. [91] M. Agarwal, J. Tardio, S. Venkata Mohan, Effect of pyrolysis parameters on yield and composition of gaseous products from activated sludge: towards sustainable biorefinery, Biomass Conver Bioref 5 (2015b) 227e235.

22

1. SUSTAINABLE HYDROGEN PRODUCTION: AN INTRODUCTION

[92] R.M. Navarro, M.C. Sanchez-Sanchez, F. Alvarez-Galvan, D. Valle, J.L.G. Fierro, Hydrogen production from renewable sources: biomass and photocatalytic opportunities, Energy Environ. Sci. 2 (2009) 35e54. [93] R.C. Saxena, D. Seal, S. Kumar, H.B. Goyal, Thermo-chemical routes for hydrogen rich gas from biomass: a review, Ren. Sus. Energy Rev. 12 (2008) 1909e1927. [94] M. Balat, Thermochemical routes biomass-based hydrogen production, Energy Sour. Recover. Utilization, Environ. Eff. 32 (15) (2010) 1388e1398. [95] M. Momirlan, T.N. Veziroglu, Current status of hydrogen energy, Ene. Edu. Sci. Technol. 6 (2002) 141e179. [96] W. Blackader, E. Rensfelt, Synthesis gas from wood and peat: the mino process, in: A.V. Bridgwater (Ed.), Thermochemical Processing of Biomass, Butterworth, London, 1984, pp. 137e149. [97] F. Liu, B. Fang, Optimization of biohydrogen production from biodiesel wastes by Klebsiella pneumoniae, Biotech. J. 2 (2007) 374e380. [98] S.V. Ginkel, S. Oh, B.E. Logan, Biohydrogen gas production from food processing and domestic wastewaters, Int. J. Hydrogen Energy 30 (2005) 1535e1542. [99] S. Venkata Mohan, G.N. Nikhil, P. Chiranjeevi, C.N. Reddy, M.V. Rohit, A.N. Kumar, O. Sarkar, Waste biorefinery models towards sustainable circular bioeconomy: critical review and future perspectives, Bioresour. Technol. 215 (2016) 2e12. [100] S. Venkata Mohan, V. Lalit Babu, P.N. Sarma, Anaerobic biohydrogen production from dairy wastewater treatment in sequencing batch reactor (AnSBR): effect of organic loading rate, Enz. Microb. Technol. 41 (2007) 506e515. [101] G.D. Saratale, S. Chen, Y. Lo, J.L.G. Saratale, J. Chang, Outlook of biohydrogen production from lignocellulosic feedstock using dark fermentation- a review, J. Sci. Ind. Res. 67 (2008) 962e979. [102] K. Nath, D. Das, Hydrogen from biomass, Curr. Sci. 85 (2003) 265e271. [103] I. Ntaikou, G. Antonopoulou, G. Lyberatos, Biohydrogen production from biomass and wastes via dark fermentation: a review, Waste. Biomass. Valor. 1 (2010) 21e39. [104] B.E. Rittmann, Opportunities for renewable bioenergy using microorganisms, Biotechnol. Bioeng. 100 (2008) 203e212. [105] M.J. Cuetos, X. Gomez, A. Escapa, A. Moran, Evaluation and simultaneous optimization of bio-hydrogen production using 32 factorial design and the desirability function, J. Power Sources 169 (2007) 131e139. [106] X. Gomez, A. Moran, M.J. Cuetos, M.E. Sanchez, The production of hydrogen by dark fermentation of municipal solid wastes and slaughterhouse waste: a two-phase process, J. Power Sources 157 (2006) 727e732. [107] S. Dahiya, A.N. Kumar, J.S. Sravan, S. Chatterjee, O. Sarkar, S. Venkata Mohan, Food waste biorefinery: sustainable strategy for circular bioeconomy, Bioresour. Technol. 248 (2018) 2e122. [108] R.K. Goud, O. Sarkar, S. Venkata Mohan, Regulating biohydrogen production by prolonged application of heat shock pretreatment facilitate selective enrichment of Clostridium sp. Int. J. Hydrogen Energy 39 (2014) 7572e7586. [109] J. Wang, W. Wan, Factors influencing fermentative hydrogen production: a review, Int. J. Hydrogen Energy 34 (2009) 799e811. [110] S. Venkata Mohan, V. Lalit Babu, P.N. Sarma, Effect of various pretreatment methods on anaerobic mixed microflora to enhance biohydrogen production utilizing dairy wastewater as substrate, Bioresour. Technol. 99 (2008b) 59e67. [111] S. Venkata Mohan, R. Kannaiah Goud, Pretreatment of biocatalyst as viable option for sustained production of biohydrogen from wastewater treatment, in: A. Mudhoo (Ed.), Biogas Production: Pretreatment Methods in Anaerobic Digestion, John Wiley & Sons, Inc, Hoboken, NJ, USA, 2012b. [112] H. Zhu, M. Beland, Evaluation of alternative methods of preparing hydrogen producing seeds from digested wastewater sludge, Int. J. Hydrogen Energy 31 (2006) 1980e1988. [113] R.K. Goud, K. Arunasri, D.K. Yeruva, K.V. Krishna, S. Dahiya, S. Venkata Mohan, Impact of selectively enriched microbial communities on long-term fermentative biohydrogen production, Bioresour. Technol. 242 (2017) 253e264. [114] International Energy Agency (IEA), Biofuels for Transport: An International Perspective, 2004. [115] A.E. Farrell, R.J. Plevin, B.T. Turner, A.D. Jones, M.O. Hare, D.M. Kammen, Ethanol can contribute to energy and environmental goals, Science 311 (2006) 506e508. [116] E.R. Orskav, W.P. Flatt, P.W. Moe, Fermentation balance approach to estimate extent of fermentation and efficiency of volatile fatty acid formation in ruminants, J. Dairy Sci. 51 (1968) 1429e1435.

REFERENCES

23

[117] I. Yacoby, S. Pochekailov, H. Toporik, M.L. Ghirardi, P.W. King, S. Zhang, Photosynthetic electron partitioning between [FeFe]-hydrogenase and ferredoxin: NADPþ -oxidoreductase (FNR) enzymes in vitro, PNAS 108 (2011) 9396e9401. [118] S. Venkata Mohan, S. Veer Raghuvulu, G. Mohanakrishna, S. Srikanth, P.N. Sarma, Optimization and evaluation of fermentative hydrogen production and wastewater treatment processes using data enveloping analysis (DEA) and Taguchi design of experimental (DOE) methodology, Int. J. Hydrogen Energy 34 (2009) 216e226. [119] V.V. Idania, R. Sparling, D. Risbey, R.S. Noemi, M. Hec, H.M. Poggi Varaldo, Hydrogen generation via anaerobic fermentation of paper mill wastes, Biores. Technol. 96 (2005) 1907e1913. [120] D.V. Gustavo, A.M. Felipe, L.R. Antonio de, R.F. Elías, Fermentative hydrogen production in batch experiments using lactose, cheese whey and glucose: influence of initial substrate concentration and pH, Int. J. Hydrogen Energy 33 (2008) 4989e4997. [121] P.S. Babu, O. Sarkar, S. Venkata Mohan, Upscaling of biohydrogen production process in semi-pilot scale biofilm reactor: evaluation with food waste at variable organic loads, Int. J. Hydrogen Energy 39 (2014) 7587e7596. [122] S.B. Pasupuleti, S. Venkata Mohan, Single-stage fermentation process for high-value biohythane production with the treatment of distillery spent-wash, Bioresour. Technol. 189 (2015) 177e189. [123] G. Mohanakrishna, S. Venkata Mohan, P.N. Sarma, Utilizing acid-rich effluents of fermentative hydrogen production process as substrate for harnessing bioelectricity: an integrative approach, Int. J. Hydrogen Energy 35 (2010) 3440e3449. [124] G. Mohanakrishna, S. Venkata Mohan, P.N. Sarma, Bio-electrochemical treatment of distillery wastewater in microbial fuel cell facilitating decolorization and desalination along with power generation, J. Haz. Mat 177 (2010c) 487e494. [125] T.V. Laurinavichene, B.F. Belokopytov, K.S. Laurinavichius, A.N. Khusnutdinova, M. Seibert, A.A. Tsygankov, Towards the integration of dark- and photo-fermentative waste treatment, Int. J. Hydrogen Energy 37 (2012) 8800e8810. [126] J.A. Modestra, M.L. Babu, S. Venkata Mohan, Electro-fermentation of real-field acidogenic spent wash effluents for additional biohydrogen production with simultaneous treatment in a microbial electrolysis cell, Sep Purif. Technol. 150 (2015) 308e315. [127] M.V. Reddy, K. Amulya, S. Venkata Mohan, Bacterial synthesis of polyhydroxyalkanoates using dark fermentation effluents: comparison between pure and enriched mixed cultures, Eng. Life Sci. 15 (2015) 646e654. [128] K. Amulya, M.V. Reddy, S. Venkata Mohan, Acidogenic spent wash valorization through polyhydroxyalkanoate (PHA) synthesis coupled with fermentative biohydrogen production, Bioresour. Technol. 158 (2014) 336e342. [129] G.V. Subhash, S. Venkata Mohan, Deoiled algal cake as feedstock for dark fermentative biohydrogen production: an integrated biorefinery approach, Inter J. Hydrogen Energy 39 (2014) 9573e9579. [130] P. Chiranjeevi, S. Venkata Mohan, Diverse acidogenic effluents as feedstock for microalgae cultivation: dual phase metabolic transition on biomass growth and lipid synthesis, Bioresour. Technol. 242 (2017) 191e196. [131] S. Omprakash, C. Sulogna, S. Venkata Mohan, Acidogenic outlet from biohydrogen reactor as phosphate solubilizing agent for integrated organic farming, J. Cleaner Production (2018), https://doi.org/10.1016/ j.jclepro.2018.09.213.